The present invention relates generally to an adaptive array antenna. More specifically, the invention relates to a radio communication system, a radio base station for use in the radio communication system, and an adaptive array antenna for use in the radio base station.
At present, the development of technologies for inexpensively constructing channels directly to subscribers using a radio transmission called a wireless local loop (WLL) has been started. Among these technologies, the form of a system capable of housing a plurality of terminal stations with respect to one base station is called a point-to-multipoint (PTMP). FIG. 1 is an illustration of a WLL in this PTMP form.
Usually, in the PTMP, a base station uses an antenna having a relatively large half angle of 60 degrees to 120 degrees since it is required to communicate to a plurality of terminal stations in different directions viewed from the base station. On the other hand, the terminal stations generally use an antenna having a small half angle of about 10 degrees and a large gain. Therefore, in the PTMP, when a base station receives, the interference from other base stations than a desired terminal station causes serious problems. FIG. 2 shows the incoming status of interference waves when a sector antenna having a half angle of 120 degrees in used. In particular, during a call to a base station, a control channel becomes a random access system wherein the base station can not carry out scheduling. Therefore, there is every possibility that many interference waves are generated, so that there is some possibility that the control channel can not accept calls to make communication impossible.
Therefore, when a usual sector antenna is used, there is some possibility that a signal transmitted from a terminal having a sharp directional antenna arrives at a very far base station, so that it is required to insure the distance for the frequency repetition. Specifically, frequency is reused by setting one unit of about 4 cells to about 7 cells, dividing frequency channels in this unit and carrying out the frequency repetition every unit. FIG. 3 shows the incoming status of interference waves in the case of the four cell frequency repetition. However, in this case, there is a limit to the repeated use of frequency, and there is a limit to the frequency channels assigned to the system, so that there is a disadvantage in that the capacity of subscribers capable of being housed by the system as a whole is limited to a small capacity.
In order to avoid interference without the need of the frequency repetition or by decreasing the number of repetitions, it has been studied to use an adaptive array antenna for directing the null direction of the antenna to another interference station, with an interference canceller for removing signals from the interference station from the original received signals by the signal processing.
However, as the control channel of the PTMP system, interference signals are generated at random timing which can not be predicted, and the duration of the interference signals is very short, i.e., in the range of from several micro seconds to tens micro seconds. Therefore, in order to sequentially detect the interference waves from the terminal and the incoming direction thereof by the base station for the terminal to carry out a control using a digital signal processing for directing the null with respect to the direction of the terminal, there is a problem in that a very high signal processing speed is required.
In addition, as shown in FIG. 4, in the case of a digital beam forming (DBF) adaptive antenna which has been mainly studied in recent years, if the transmission rate increases to 1 Mband or more which has been studied in the PTMP system, there is a problem in that a very high digital signal processing must be carried out in order to carry out a real-time receiving.
On the other hand, also in the case of the PTMP system similar to mobile communication, in order to inhibit undesired interference waves from being generated, it is considered to control a transmitted power from a mobile station to substantially fix a received power at a base station. However, also in this case, there are some cases where the actual received power can not be constant due to the influence of fading and shadowing. Therefore, in order to substantially fix the signal level at the final stage of a receiver, a base station including an adaptive array antenna must have an automatic gain control (AGC) function. For example, as shown in FIG. 5, it is considered to provide the AGC function by inserting a variable gain amplifier 3801 at the output after combining the adaptive array antennas. However, for example, when signals from terminals other than a desired terminal are stopped in a cell to continuously change a phase-shifted amount by means of a phase shifter to scan the null point, it is predicted that the dynamic range of the signal level after combining is very large, whereas the strength of signals from each of antennas before combining substantially has the same level. In this case, as shown in FIG. 5, if the gain of the variable gain amplifier 3801 for AGC provided in the signal line after combining is raised in accordance with the decrease of the level of the received signal after combining, there is a problem in that a part of the flow of the signals before combining is saturated.
To the contrary, after the direction of the terminal can be substantially identified, or after the weighting coefficient substantially converges at the optimum weighting coefficient, when beams are combined so as to be directed to that direction, the signal strength after combining is stable so that the variation in strength is small. On the other hand, there are some cases where the level of signals from each of the antennas before combining is increased by the combining of signals from a plurality of terminal stations. In this case, since the AGC function is hardly operated, there is a problem in that a part of the flow of the signal before combining is saturated.
In addition, when an adaptive array antenna is used for transmission, if transmitted power control is carried out by only a variable gain amplifier 3901 before division to each of elements as shown in FIG. 6, or if transmitted power control is carried out by only a plurality of variable gain amplifiers 4001 provided in signal paths for each of the elements after division as shown in FIG. 7, there is a problem in that the effective radiation power (ERP) taking account of the directional gain of the adaptive array antenna exceeds a legal limit, or high frequency circuit elements for individual elements are saturated.
As described above, in the prior art, there is a problem in that it is required to provide a very high signal processing speed if an adaptive array antenna is used for reducing interference from terminal stations in the PTMP.
In addition, in the case of the digital beam forming (DBF) adaptive array antenna which has been mainly studied in recent years, if the transmission rate increases, there is a problem in that it is required to carry out a very rapid digital signal processing in order to carry out a real time receiving.
In addition, when the AGC function is provided by inserting a variable gain amplifier in the output after combining the adaptive array antenna, if the gain of the AGC amplifier is increased in accordance with the decreased level of the received signal after combining, there is a problem in that a part of the flow of the signal before combining is saturated.
Moreover, when an adaptive array antenna is used for the base station transmission, if the transmitted power is controlled by only the variable gain amplifier before the division to each of the elements or if the transmitted power is controlled by only the plurality of variable gain amplifiers provided in the signal paths to each of the elements after the division, there is a problem in that there are some cases where the effective radiation power taking account the directional gain of the adaptive array antenna exceeds a predetermined value, or the high frequency circuit elements for individual elements are saturated.
It is therefore an object of the present invention to eliminate the aforementioned problems and to provide an adaptive array antenna capable of carrying out a real number weight control based on the maximum diving method by deriving a differential coefficient of the performance function with respect to a real number weight using a plurality of individual element signal strengths, which are detected by individual element signal strength detecting means, and a combined signal strength which are detected by combined signal strength detecting means, the adaptive array antenna being capable of realizing a simper circuit construction than that in the prior art wherein the demodulated signal of each of antenna elements is used.
It is another aspect of the present invention to provide an adaptive array antenna comprising a plurality of antenna elements, a plurality of high-frequency circuits, each of which is connected to a corresponding one of the antenna elements, and a high-frequency combining circuit for combining the outputs of the plurality of high-frequency circuits, the adaptive array antenna being capable of controlling the output signal level after combining to be within a predetermined range and of preventing the high-frequency circuits for the respective individual elements from being saturated.
It is a further object of the present invention to an adaptive array antenna comprising a plurality of antenna elements, a plurality of high-frequency circuits, each of which is connected to a corresponding one of the antenna elements, and a high-frequency dividing circuit for dividing outputs to the plurality of high-frequency circuits, each of the high-frequency circuits having a weight control circuit for weighting amplitude or phase of each of the antenna elements, the adaptive array antenna capable of controlling so that an effective radiation power taking account of the directional gain of each of the antenna elements does not exceed a predetermined value, and of controlling so that the high-frequency circuits for the respective individual elements are not saturated.
In order to accomplish the aforementioned and other objects, according to a first aspect of the present invention, an adaptive array antenna comprises: a plurality of antenna elements; a plurality of weighting means for weighting received signals, which are received by said antenna elements, by weights which are set, respectively; combining means for combining the received signals weighted by said plurality of weighting means; signal strength detecting means for detecting the strength of the received signal combined by said combining means; and weight control means for calculating a weight on the basis of the strength of the received signal detected by said signal strength detecting means, and for setting the calculated weight in each of said plurality of weighting means, wherein said weight control means comprises: a changing part for changing the weight which is set in one of said plurality of weighting means; and a setting part for calculating a weight on the basis of the variation in strength of the received signal detected by said signal strength detecting means when said weight is changed by said changing part, and for setting the calculated weight in said one of said plurality of weighting means.
According to a second aspect of the present invention, an adaptive array antenna comprises: a plurality of element antennas; a plurality of high-frequency circuits, each of which is connected to a corresponding one of said element antennas; and a local signal phase-shifting circuit for varying the phase of a local signal, which is added to a frequency converting circuit in said high-frequency circuit, every one of said high-frequency circuits for each of said element antennas, wherein each of said plurality of high-frequency circuits has a coupler for branching a part of signals from each of said element antennas, and a quadrature demodulator for an individual element antenna, to which signals are inputted from said coupler.
According to a third aspect of the present invention, an adaptive array antenna comprises: a plurality of antenna elements; a plurality of high-frequency circuits, each of which is connected to a corresponding one of said antenna elements; a high-frequency combining circuit for combining the outputs of said plurality of high-frequency circuits; at least one first RSSI circuit for monitoring at least one signal level of RF or IF signals from a plurality of individual antenna elements; a second RSSI circuit for monitoring the signal level of the combined RF or IF signal from said high-frequency combining circuit; (Nxe2x88x921) first variable gain circuits for allowing the variation in relative levels of all of RF or IF signals of each of individual elements; a second variable gain circuit capable of varying the signal level of the RF or IF signal from high-frequency combining circuit; and a gain control circuit for controlling the output signal level after combining to be within a predetermined range on the basis of RSSI signals from said first RSSI circuit and second RSSI circuit, and for controlling said first variable gain circuit and said second variable gain circuit so as to prevent a high-frequency circuit element for each of said individual elements from being saturated. In the above sentence, the term RSSI is an abbreviation of a received signal strength indication, which is a numerical value of the strength of en electric wave signal during receiving.